The invention is generally directed to offshore floating structures used in the production of oil and natural gas and particularly to semi-submersible structures.
There are a variety of floating structures used in the production of oil and gas in deeper water offshore. Each type of structure has its own advantages and disadvantages relative to motion characteristics that may make it more or less suitable for use in certain wave conditions.
The semi-submersible is a type of floating structure that has vertical columns supporting topsides, with the columns being supported on large pontoons that extend between the columns as seen in FIGS. 14A and 14B. The structure typically is held in position by the use of spread mooring lines that are anchored to the seafloor. The semi-submersible has a number of unique characteristics compared with other floating structures such as a Spar and TLP (tension leg platform).
Conventional semi-submersible structures provide general advantages that include: the semi-submersible has good stability because of a large foot-print and low center of gravity. The hull requires lower steel tonnage. The semi-submersible may include drilling capability. The semi-submersible can support a large number of flexible risers or SCRs (steel catenary risers) because of the space available on the pontoons. The topsides can be integrated at quayside and thus reduce cost and save scheduling time. The semi-submersible has a relatively short to medium development schedule and the initial investment is relatively low. The semi-submersible can also be held at a relative shallow draft during launch and fit up work, which means that it is capable of being launched or worked upon at quayside adjacent most construction yards worldwide. The semi-submersible provides a larger payload capacity than Spars and can operate in deeper water than TLPs. Semi-submersibles allow quay-side integration and are simpler to install than both Spars and TLPs.
The semi-submersible also has several deficiencies. The most significant is that rougher water created by storms can cause large heave (vertical) motions. As a result, semisubmersibles have not been suitable for a dry tree riser arrangement. A dry tree riser arrangement has the well controls (referred to in the industry as the “tree” or “Christmas Tree”) above the water line on the vessel. The flow connection between the seabed and the dry tree is provided by a vertical top-tension riser (TIR). The dry tree riser arrangement has significant economic benefit for well completion, work-over, and intervention during the life of the offshore production facility. The dry tree riser also offers the operational advantages of flow assurance, well access, drilling, etc., which is not possible with wet tree units.
The offshore industry has been attempting to develop a successful arrangement for a dry tree semi-submersible as an alternative to Spars and TLPs for more than a decade. That effort has been unsuccessful so far. Another problem from the large heave motion is that it causes fatigue in SCRs more easily, which requires more stringent fatigue design for the SCRs and thus costs more. For a platform with large diameter SCRs, the solutions to this problem could become technically or economically unfeasible.
The ideas that have been explored by the industry to achieve low motion characteristics of semi-submersibles generally fall into the categories below.
The first is a deep draft semi-submersible. The concept is to increase the draft from the normal range from sixty to eighty feet to greater than one hundred feet so that the wave action at the keel is reduced and, thus, the structure will have less motion. This makes the semi-submersible option feasible in some locations where the conventional semi-submersible would not be chosen because of the difficulties in dealing with the SCR riser fatigue issues. However, the heave motion is still relatively large compared with spars and TLPs. Also, the dry tree arrangement is still not feasible.
The second is a semi-submersible with one or more heave plates 48 situated below the hull. This is illustrated in FIG. 13. The basic idea is to add a heave plate or pontoon at the keel that extends in deep draft. The heave plate or pontoon adds damping and added mass to the platform which will reduce its heave motion under wave conditions.
Most concepts based on the heave plate have the heave plate or pontoon as an extendable part attached to the bottom of the semi-submersible hull by means of columns or a truss structure. The heave plate or pontoon is retracted at the fabrication yard and during transportation. After the hull is located on the site, the heave plate or pontoon is then extended or lowered to a deeper elevation and locked at that position.
The known designs suffer several deficiencies. The extendible columns take too much deck space. In some cases it could be as much as thirty percent of the total deck space, which is impractical from a topsides equipment layout point of view. The structural connections and locking mechanisms of extendible columns are complicated. They are hard to build, risky during installation, and difficult to maintain, On the other hand, designs with rigidly attached heave plates have a much greater draft than a conventional semi-submersible and cannot be readily brought quayside.
The desired features of an alternative to the Spar and TLP are: 1) motion characteristics compatible with TTRs, 2) low cost, 3) ability to operate in water depths exceeding 8,000 feet, 4) large deck area, 5) high payload capacity, and 6) quay-side integration/commissioning.
The challenge for semi-submersible structures as dry-tree floaters is their comparatively large heave response in waves. Since dry-tree risers are arranged vertically, the relative motions between vessel and risers must be compensated by riser tensioners. Typical semi-submersible designs have a heave response resulting in a tensioner stroke that exceeds the stroke range of existing riser tensioners. Achieving a heave response compatible with market-ready tensioner technology is therefore crucial for developing a dry-tree semisubmersible.